There are two varieties of stamps, differing from the normal through some slip in the process of manufacture--bicoloured stamps, in which the portion printed in one colour is inverted as regards the remainder of the design, caused by carelessness in "feeding" the partly-printed sheet wrong way up into the press, for the second impression completing the design; and pairs of stamps, which, each quite normal if severed, are when _se tenant_ inverted in respect to each other, a condition philatelically termed _tête-bêche_.
The fraudulent manipulator has turned his attention to these, generally scarce and frequently very rare, eccentricities, cutting out from the bicoloured stamp the part printed in one colour and replacing it with great care, but upside down; and, as to the _tête-bêche_ pairs, manufacturing them by means of two single copies, a strong adhesive mixture and heavy pressure.
Sometimes, so well have these frauds been made that nothing short of several hours' _boiling_ has sufficed to dissolve the illegal union of the two pieces of paper--a drastic test, and one somewhat detrimental to the value of such copies as are enabled, by their genuineness, to survive the ordeal. The possible result to, say, a mint imperforate Fourpence, Ceylon, suspected of having recently acquired its otherwise desirable "margins," reminds me of the test given (not advocated) by a famous philatelist for the detection of forgeries of early Cashmere stamps, which were printed in water-colour--"Put them in water; if the colour is 'fast' the stamp is a forgery; if it comes off, leaving a blank piece of paper, the stamp is genuine"!
A famous forgery was put on the market some years ago, the stamp imitated being the One Penny value of the well-known first issue of New South Wales, commonly called "Sydney Views." This stamp was issued in sheets of twenty-five, each repetition of the design being separately engraved on the plate and so giving twenty-five minor varieties; and subsequently the entire plate was re-cut, doubling the number of varieties for the specialist. The forger engraved his fraudulent wares and printed the labels, as were the originals, direct from the plate, in a very good imitation of the ink used in 1850 and on similar paper; and these reproductions, often in pairs, were affixed to old envelopes and cancelled with forged postmarks.